There are occasions where an isolated enclosed space must be used even though the atmosphere surrounding the space becomes polluted with contaminants noxious to personnel or damaging to equipment in the space. Contaminants may be considered noxious based on corrosiveness, radioactivity or high temperature. Examples of environments which may be susceptible to noxious contaminants include armored tanks, collapsible personnel shelters, and personnel carriers. Also, oil and recovery injection wells are susceptible to hydrogen sulfide gas leakage which may present a hazard to individuals and machinery in the vicinity of the well.
Protection of an enclosed space, such as those listed above, from noxious contaminants, is usually accomplished by providing a filtering unit for the space. In the case of nuclear particles, biological agents and noxious chemicals, these filters generally consist of at least three stages of filtration: first, a roughing filter stage to remove the bulk of the dust entrained in the incoming air; second, a Hepa filter stage to remove biologic agent and any liquid or solid chemicals; and finally, an activated charcoal filter to remove gaseous noxious chemicals.
In the case of war machines such as armored tanks or personnel shelters, normal ventilation of the habitable space of the tank or shelter is provided by a blower, which is capable of raising the pressure of the interior space so that air leakage is outward and agent anti-penetration integrity is preserved. It is most important to the integrity of the filtering system that any possible leaks in the filter unit are from higher pressure clean air to lower pressure dirty air.
When going from the normal ventilation mode to a protective operating mode, a by-pass valve or series of valves is required to shut the normal ventilation air passageway tight and to open air inlet and outlet passageways for the filtering unit. The change from the ventilation mode to a protective filtering mode must be done quickly and reliably in extremely hostile environments.
Prior protective filtering systems consist of one or two flapper valves, which are closed against the filter unit inlet and outlet passages, sealing them and the filter from contaminants present in the normal ventilation air. On demand, these valves are forced to opposing seats in the by-pass ventilation air passageway, closing it and opening passages to allow the filtering unit to operate. This type of flapper valve has been subject to problems in that the seats and flappers, which must close air tight reliably, are exposed to dust, dirt, moisture and humidity which may be present in the normal ventilation air; such exposure may result in the by-pass valve failing to shut reliably. Sealing an environment through the use of flapper-type valves has been made somewhat more reliable by pressurizing the space between the two flappers with cleaned air from the interior pressurized habitable space, thus if the upstream valve does not seal perfectly, the leak will be from the space between the valves to the dirty air side. However, this is only the case when the leak is small. Larger obstructions on both sealing surfaces for upstream and downstream valves would overcome the pressurized flow resulting in a leak in the system.
Providing a by-pass valve which assures the integrity of the filtering mode in extremely noxious environments is highly desirable. The present invention meets the requirements of high reliability and advances the state of the art beyond that which is known at this time.